Calibrating focus on a Nikon FM

[Bob-D659]

Well you have to adjust the screen and the mirror so when a square subject is square on the film plane, it is square on the screen and in focus. Not to mention centered on both the film plane and the screen. Hopefully the mirror is right at the moment.

[Hops]

OK, I found a service manual for the FM2. It's pretty similar.

They have you first adjust the screen roughly by shimming the four screws that hold the screen/prism cage in place, then adjust the mirror for fine adjustment. I'd rather not move the mirror any more because it's probably already been fine-adjusted, and so further movements could be even more off from 45 degrees.

I discovered that if I leave the four washer off, I can simply adjust the screws up and down, and the spring tension that holds the focus screen in place pushes up against the heads of the screws. So I can just tweak these screws until it's calibrated, then find washers that fit in the gaps. I might just epoxy them in place once I have it perfect, and have shot a test roll to confirm.

[Diapositivo]

Hops, be careful with epoxy, glues can create a vapour that might fog the screen. This can happen years away from the application of the glue. I would avoid any unnecessary glue, or use a very specific one.

[Hops]

BTW, for those interested...

For aligning the mirror, the goal is to get it at exactly 45 degrees to the focus screen. (later fine tuning it for focus) Nikon's service manual shows that they use a special attachment with a collimator shining into the lens mount. There is also a mirror that goes on top of the focus screen. If the mirror is at 45 degrees, the light will shine straight back where it came from. I wonder if this could be done with a laser pointer instead of a collimator?

[Bob-D659]

Yes, if you can align the laser pointer beam to be at exactly 90 degrees to the lens mount flange.

[Mark Crabtree]

I guess the central premise of my issue is...

Does a ground glass on the film rails give you a different result than actual film?

I think (but am not certain) there is a slight difference due to the film laying against the pressure plate rather than against the front rails. I've been sorting out some issues with my Nikon F's and lens for a few days and have done the ground glass comparison indoors and out with several lenses on three bodies. I find the gg puts the lens at a very, very, slightly closer distance than the viewfinder (3 bodies). The viewfinder seems correct.

I guess that the right way to do it would be with a collimator checking the image at the film plane. Nonetheless, I think you could use the gg to get the focus close and the screen aligned properly to the film plane and plane of focus (the harder task). Then check with a roll of film with bracketed focus. The inclined focus target is a good start, but only checks one point.

I put a small piece of drafting tape on the lens to mark the focus position. Then bracket about the thickness of the focus indicator line, then that much again (you want one to go far enough that things get worse so that you know you went far enough). Both directions, but I think the error will be such that you only need to try farther focus than the gg indicates (it really can't go the other way unless the film were bulging forward).